Seawater typically contains about 4 to 7 ppm boron, in addition to a variety of water-soluble salts. Traditional methods for purifying (desalinating) seawater for drinking and irrigation purposes utilize reverse osmosis (RO) membranes, which are effective at significantly reducing the concentrations of all dissolved ions in the seawater. Although the reduction of the majority of dissolved ions by polyamide reverse osmosis membranes is about 98% to about 99%, the rejection rate of boron by these membranes is much lower, typically in the 70%-90% range, and may be even lower at high feed water temperatures (greater than about 25° C.).
The significantly lower rejection rate of boron by polyamide membranes may be explained by the very low dissociation rate of boric species at neutral pH. However, this boric species dissociation rate increases with pH and reaches 50% dissociation at a pH of 8.6 to 9.8, depending on the ionic strength of the solution and the temperature (W. Stumm, et al. Aquatic Chemistry, John Wiley & Sons (1981)). Consequently, an increased boron rejection rate is achievable at high pH, thus making possible appreciable reduction of boron concentration by reverse osmosis.
Magara et al. (Desalination 118:25-34 (1998)) and Prats et al. (Desalination 128: 269-273 (2000)) describe methods for reducing boron concentration using two-pass reverse osmosis systems. In these systems, the pH of the permeate from the first pass is increased before it is passed through the RO membrane in the second pass in order to improve the boron rejection. The term “permeate” is known in the art to refer to reverse osmosis product water. Because the RO permeate from these systems has low salinity and low concentration of scale-forming ions, even adjustment of the pH to high levels does not result in scale formation.
An example of a similar methodology applied to high salinity water is described by Tao et al. (U.S. Pat. No. 5,250,185), which involves the application of a high pH RO processing method to oilfield-produced water. In order to prevent scaling of the reverse osmosis system by carbonate salts, the feed water is softened prior to adjustment of the pH to a level greater than 9.5. Tao et al. teach that the high pH is necessary to obtain the desired increase in boron rejection. Additionally, Mukhopadhyay (U.S. Pat. No. 5,925,255) describes the treatment of brackish and low salinity water by reverse osmosis, in which the hardness of the RO feed water is removed by a weak acid cation exchange resin.
Surface seawater filtration processes typically involve in-line flocculation followed by media filtration. It has been found that for efficient flocculation using ferric flocculent, the pH of seawater should be maintained at about 7, which is below the typical native pH of about 7.8 to 8.2, since higher pH levels may have an adverse effect on the effectiveness of media filtration with flocculation of seawater. Effective flocculation may be desired prior to media filtration to provide efficient removal of colloidal material from the RO feed water and prevent RO membrane fouling.
One possible solution would be to initially reduce the pH of the seawater from about 7.8-8.2 to about 7, add flocculent, and then pass the seawater through the media filters. After media filtration and before RO processing, the pH of the filter effluent could be increased to about 8 to 9.5 to increase boron rejection by RO membranes. Such an approach would result in both effective filtration of feed water and higher boron rejection by RO membranes. However, the use of acid for pH reduction, followed by caustic for acid neutralization, would result in additional operating costs.
Systems using membrane technology, such as ultrafiltration (UF) or microfiltration (MF), for pretreating feed water prior to RO are known. For a example, a system configuration called Integrated Membrane System (IMS) produces feed water of superior quality with respect to suspended solids. Such integrated systems contain two different types of membranes which are functionally connected; the pretreatment section may use either MF or UF membranes. The IMS configuration has been tested extensively as a pretreatment for RO systems. For example, Glueckstern et al. (Proceedings of ICOM Conference, Toulouse, 2002) describe the reduction of colloidal matter in seawater prior to RO using parallel operation of ultrafiltration and media filtration systems. Similar quality of seawater effluent was obtained during operation of the two types of systems during periods of average water quality. However, during periods of stormy weather, media filters could not cope with the increasing load of suspended matter. In contrast, the ultrafiltration system produced good quality effluent, suitable for RO, regardless of the quality of the raw water.
It would be desirable to be able to significantly reduce the concentration of boron in high salinity liquids in straightforward processes that would be attractive due to lower operating costs and superior effectiveness relative to known methods.